feat: embed Chatwoot live-chat widget (destek.sase.tr)

Site-wide live-chat widget served from the self-hosted Chatwoot at
destek.sase.tr, with verified user identity and vehicle context.

- apps/web: lib/chatwoot.ts loads the SDK lazily (mirrors the PostHog
  init pattern), init in main.tsx, identify logged-in users in __root
  via a server-computed HMAC, and attach the viewed vehicle (VIN/brand/
  model) as contact custom attributes on the vehicle detail page.
- apps/api: GET /api/chatwoot/identity (AuthGuard-protected) returns
  HMAC-SHA256(user.id) so the widget can use verified identity.
- env: VITE_CHATWOOT_BASE_URL + VITE_CHATWOOT_WEBSITE_TOKEN (build-time,
  wired through docker-compose.coolify.yml build args + Dockerfile ARG)
  and CHATWOOT_HMAC_TOKEN (api runtime). All optional — widget and
  endpoint no-op when unset.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -28,6 +28,8 @@ ARG VITE_FARO_ENABLED=false
 ARG VITE_FARO_COLLECTOR_URL=
 ARG VITE_POSTHOG_KEY=
 ARG VITE_META_PIXEL_ID=
+ARG VITE_CHATWOOT_BASE_URL=
+ARG VITE_CHATWOOT_WEBSITE_TOKEN=
 
 RUN pnpm build